1. Field of the Invention
The present invention relates to improvements in a coil device for use in a flyback transformer, a switching power transformer, a choke coil or the like and more particularly it relates to improvements in a coil device employing a magnetic core with a gap.
2. Description of the Prior Art
In any of the conventional transformers, choke coils and so forth known heretofore, it is customary to form a gap in a closed magnetic path so that the magnetic core thereof is not saturated when a desired current is caused to flow. For example, when a ferrite magnetic core usually having a magnetic permeability .mu. of 5000 or so is used in a transformer, a gap is formed there to reduce the effective permeability .mu. within a range of 50 to 300.
This signifies that a gap having a great magnetic reluctance needs to be existence in a ferrite magnetic core of which magnetic reluctance is originally small, wherein a great leakage flux is generated in the periphery of the gap.
It is generally known that such leakage flux exerts at least two harmful influences as follows.
(1) Noise is induced in peripheral apparatus (components) which are prone to be effected by magnetic induction. PA1 (2) In case the coil is so wound as to surround the gap, there occurs abnormal generation of heat in the coil around the gap due to the leakage flux.
For the purpose of solving the above problems, a variety of improvements have been developed.
In an attempt to settle the problem (1) above, there is contrived a coil device 1' of forming a gap merely in the coil alone. In FIG. 17 is illustrated a structure of this conventional type of coil device 1'.
This coil device 1' is constructed such that a sectionally U-shaped first magnetic core 2' is combined with a similarly sectionally U-shaped second magnetic core 3' and then a coil 6' is wound around portions of the magnetic cores 2' and 3'.
The first magnetic 2' and the second magnetic core 3' have legs 2a' and 3a', respectively. The first magnetic core 2' and the second magnetic core 3' are arranged such hat the first leg 2a' and the first leg 3a' are oppositely faced to each other via a gap 5'. The coil 6' is wound so as to cover the gap 5' within it. The opposing legs 2a' and 3a' are formed into such a shape as one in which their lateral sectional areas become equal to each other over their entire lengths. As a combination of the magnetic cores, there may be another sectionally E-shaped core.
A B-H curve shown in FIG. 18 shows a data found in the prior art coil device 1'. As shown in this figure, a maximum flux density Bm of the conventional type of coil device 1' is 5510 Gs.
Table 1 below indicates a result of measurement of temperatures in a coil center X, a coil end Y, a core Z and a periphery W of the conventional type of the coil device 1' measured by a testing device T' shown in FIG. 16B (Test condition: Frequency 80 kHz, Sine wave of 1.0A and Ambient temperature of 40.degree. C.).
TABLE 1 (.degree. C.) X Coil Y Z W Structure center Coil end Core Periphery Prior Art 107.5 79.0 74.5 44.0
As indicated in Table 1, a mere arrangement of the gap 5' only in the coil 6' causes a high temperature of more than 100.degree. C. at the coil center X and further the problem (2) above is expanded more.
As regards the problem (2) above, as already disclosed in Japanese Patent Laid-Open No. 55-77115 and Japanese Utility Model Laid-Open No. 57-130402, this problem is resolved by a method wherein the gap placed within the coil is divided magnetically into a series of plural segments so as to disperse a concentration of leakage magnetic flux. In addition, there are Japanese Utility Model Publication No. 53-53850 and Japanese Utility Model Publication No. 60-7448 in order to resolve the problems (1) and (2) above. These utility models use material as a gap filler of a material having a large specific permeability than that of air (more than 1), reduce magnetic reluctance at the gap and further decrease the leakage magnetic flux.
As described above, in case that the material quality having a greater relative permeability than that of air (more than 1) is arranged within the coil as the gap member, there is a possibility that the problems (1) and (2) above can be improved to a certain degree.
However, even in this case, there remains a problem that a leakage magnetic flux may be concentrated at an interface part between the gap and the magnetic core. In addition, there is a new problem that it is hard to get such material as one in which it has an appropriate permeability as the gap filling material, a high saturated magnetic flux density and a low magnetic core loss characteristic corresponding to the magnetic core. Due to this fact, this system may generate the following new problem. Namely, the coil wound over the interface part between the gap and the magnetic core may generate heat abnormally. In addition, the gap may also generate heat abnormally due to the loss of magnetic core at the gap filling material. Further, the B-H curve of the magnetic core having the gap filling material therein becomes non-linear form and if this is used in a transformer, it may produce a deformed wave form. This is the present state that a more effective improvement may not be attained.
It is therefore an object of the present invention to provide a coil device capable of resolving the aforesaid problems, reducing influence of noise against the peripheral apparatus (component), reducing a leakage magnetic flux generated around the gap and preventing an abnormal generation of heat in the coil. It is another object of the present invention to provide a coil device whose cost is less expensive and its reliability in operation is improved.